Composition and method for producing brown coating on brass

ABSTRACT

An aqueous bath for imparting a brown surface coating to brass workpieces is provided by an aqueous solution of cupric carbonate; a thiosulfate or hydrosulfite color modifier; an organic color intensifier; and ammonium hydroxide in an amount sufficient to provide a pH of about 10.5 to 12.5. The workpieces are immersed in the bath, preferably at ambient temperature, for a period of time sufficient to develop a uniform brown finish thereon. The color intensifiers may be aliphatic and aromatic carboxylic acids, aliphatic and aromatic aldehydes, aliphatic and aromatic ketones, aliphatic and aromatic amines, aliphatic alcohols, oxidation-resistant hydroxybenzene compounds, aromatic phosphonic and sulfonic acids, and mixtures thereof.

United States Patent i151 Brough COMPOSITION AND METHOD FOR PRODUCING BROWN COATING ON BRASS [75] Inventor: Richard W. Brough, Windsor, Conn.

[73] Assignee: Conversion Chemical Corporation, Rockville, Conn.

[22] Filed: Nov. 9, 1972 [21] Appl. No.: 304,968

[52] US. Cl. 148/624, l48/6.14 R [51] Int. Cl. C23f 7/24, C23f 5/02 [58] Field of Search l48/6.l4 R, 6.24, 6

[56] References Cited UNITED STATES PATENTS -l/l954 Heincckc 148/624 OTHER PUBLICATIONS Harris, Metal Industry, Jan. 12, 1951, pp. 23-25.

[ June 11, 1974 Primary Examiner-Ralph S. Kendall Attorney, Agent, or FirmPeter L. Costas [5 7 ABSTRACT An aqueous bath for imparting a brown surface coating to brass workpieces is provided by an aqueous solution of cupric carbonate; a thiosulfate or hydrosulfite color modifier; an organic color intensifier; and ammonium hydroxide in an amount sufficient to provide a pH of about 10.5 to 125. The workpieces are immersed in the bath, preferably at ambient temperature, for a period of time sufficient to develop a uniform brown finish thereon. The color intensifiers may be aliphatic and aromatic carboxylic acids, aliphatic and aromatic aldehydes, aliphatic and aromatic ketones, aliphatic and aromatic amines, aliphatic alcohols, oxidation-resistant hydroxybenzene compounds, aromatic phosphonic and sulfonic acids, and mixtures thereof.

21 Claims, No Drawings COMPOSITION AND METHOD FOR PRODUCING BROWN COATING ON BRASS BACKGROUND OF THE INVENTION Several baths for producing a brown coating on brass surfaces are known in the art and in general use; however each is recognized as having its own peculiar drawbacks. An aqueous sulfate bath of copper sulfate, nickel sulfate and potassium chlorate requires the use of bath temperatures of 90 Centigrade and higher, such elevated temperatures tending to promote attack on the workpiece. An aqueous sulfide bath of barium sulfide and ammonium carbonate causes smut to form on the treated surface, and thereby impairs the adhesion of any subsequently applied lacquer film. The sulfide baths are furthermore hard to control due to an inherent lack of stability over extended periods of usage.

The copending United States Patent Application of Merton M. Beckwith, Ser. No. 304,966, filed Nov. 9, l972, and assigned to the same assignee, discloses the use of an aqueous copper carbonate/color intensifierlammonia bath for the single step blackening of brass surfaces at ambient temperature. The bath is longlasting, easy to prepare and replenish, and produces a deep, highly uniform coating on the workpiece in a brief immersion time. The present invention arises out of an attempt to find a color modifier useful in the aforedescribed blackening bath which will cause the copper carbonate blackening bath to operate as a bath for producing brown deposits.

Accordingly, it is an object of the present invention to provide a novel copper carbonate-based bath for developing uniform and adherent brown coatings on brass surfaces at ambient temperatures in a brief immersion time.

It is also an object to provide such a bath which has defined operative pH, temperature, and immersion time ranges, which is easy to prepare and replenish and which is stable over a prolonged period of use.

Another object of the present invention is to provide a novel method for developing uniform and adherent brown surface coatings on brass workpieces which utilizes the baths described above.

SUMMARY OF THE INVENTION It has now been found that the foregoing and related objects may be readily obtained by providing a bath for imparting a brown surface coating to brass workpieces comprising an aqueous solution of about 35 to 135 grams per liter of cupric carbonate; about 1 to' 30 grams per 100 grams of cupric carbonate of an organic color intensifier selected from the group consisting of aliphatic and aromatic carboxylic acids, aliphatic and aromatic aldehydes, aliphatic and aromatic ketones, aliphatic and aromatic amines, aliphatic alcohols, oxidation-resistant hydroxybenzene compounds, aromatic phosphonic and sulfonic acids, and mixtures thereof; about 1 to 30 grams per liter of a color modifier selected from the group consisting of thiosulfate and hydrosulfite salts of non-interfering cations and mixtures thereof; and ammonium mmonium hydroxide in an amount sufficient to provide a pH of about 10.5 to

12.5. Preferably the solution contains about 45 to 75 grams per liter of cupric carbonate, about 15 to 25 grams of the organic intensifier per 100 grams of cupric carbonate, about 4 to 10 grams per liter of color modifier, and has a pH of about 11.2 to 12.0. The organic color intensifier is preferably a carboxylic acid, espe cially an aliphatic polycarboxylic acid such as citric acid, tartaric acid or mixtures thereof; and the color modifier is preferably a thiosulfate salt of a cation selected from the group consisting of alkali metals, ammoniumand copper.

The method of imparting a brown surface finish to brass and brass alloys is comprised of the steps of dissolving in water reagents to form the aforementioned solution, maintaining the solution at a temperature of about 20 to 50 Centigrade and immersing in the solution a workpiece having a brass surface for a period of time sufficient to develop a brown finishthereon. Preferably, the bath is maintained at a tempersture of about 25 to 30 Centigrade.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As indicated hereinbefore, the bath for the practice of the present invention is an aqueous solution of cupric carbonate, a water-soluble color intensifier, a water-soluble thiosulfate or hydrosulfite color modifier, and sufficient ammonium hydroxide to provide the desired level of alkalinity.

Although the theory of operation is not completely understood, it is believed that under process conditions the color modifier may be adding trace amounts of sulfide to the coating and thereby modifying the color produced. In any case, the brown finish provides a good surface for subsequent coatings, such as acrylic or other clear protective lacquers.

Cupric Carbonate The cupric carbonate (CuCO is typically and easily provided in the form of commercial basic copper carbonate, which contains equal molar weights of cupric carbonate and cupric hydroxide and has an average copper content of 55 percent by weight. While the cupric carbonate might be formed in situ in the bath from a copper compound and carbonic acid, means would have to be provided for removing any interfering radical from a copper compound other than cupric hydroxide and it has generally been found simpler and cheaper to use the aforementioned basic copper carbonate.

Generally 35 to 135 grams of cupric carbonate are dissolved in each liter of solution, and preferably from 45 to grams per liter. Amounts greater than grams per liter produce no additional benefits, and amounts less than 35 grams per liter produce poor coloration and require excessively long immersion times.

Color lntensifiers The water-soluble organic color intensifiers useful in the practice of the present invention are selected from the group consisting of aliphatic and aromatic carboxylic acids, aliphatic and aromatic aldehydes, aliphatic and aromatic ketones, aliphatic and aromatic amines, aliphatic alcohols, oxidation-resistant hydroxybenzene compounds, aromatic phosphonic and sulfonic acids, and mixtures thereof.

Included in the carboxylic acid class are the monocarboxylic acids (e.g., lactic, glycolic, gluconic, benzoic and nitrobenzoic acids), the polycarboxylic acids (e.g., oxalic, maleic, tartaric, malic, malonic, diglycolic, adipic, citric, ethylene diamine tetracetic, and 2-terephthaloyl benzoic acids), the acid anhydrides (e.g., succinic and phthalic anhydrides), and the amino acids (e.g., aminoacetic acid). The aldehyde class includes such members as the butyl, anisic, cinnamic and 2,4-dichlorophenyl aldehydes and maltose, while the ketone class includes such members as acetone and methyl ethyl ketone. Members of the aliphatic alcohol class include the monohydroxy members (e.g., methanol, isopropanol, and glycol ethyl ether) as well as the polyhydroxy members (e.g., glycerine). The amine class includes such members as ethylenediamine and o-toluidine, while the aromatic phosphonic and sulfonic acid class encompasses benzene phosphonic acid and toluene sulfonic acid. The oxidation-resistant'hydroxybenzene class is composed of ringhydroxy substituted compounds including phenol, resorcinol (mdihydroxybenzene'), p-hydroxybenzoic acid and salicylicacid (o-hydroxybenzoic acid) which exhibit little tendency to act as reducing agents and which are relatively active oxydizing agents compared to such hydroxybenzene compounds as pyrogallic acid (1,2,3-

trihydroxybenzene) and hydroquinone (pdihydroxybenzene) which do not act as reducing agents.

Naturally, for the purpose of determining the suitability of an aromatic compound as a color intensifier when the compound has two different functional groups and is properly classifiable both in the hydroxybenzene class and also another class, it is to be considered as a hydroxybenzene and required to meet the oxidation-resistant criterion set forth formembers of the class. It should also be understood that, for the purpose of this description of the colorintensifiers useful in the practice of the present invention, the term aliphatic has been used in its broadest sense to include generally all non-aromatic compounds and specifically to exclude aromatic compounds. Finally it should be recognized that the carboxylic acids may be introduced into the solution as salts or esters of non-interfering ions, and that the acids may react in solution with the ammonium or copper anions to produce salts.

The preferred color intensifiers are the water-soluble carboxylic acids, and of these the aliphatic polycarboxylic acids have been found most useful in the practice of the invention. Citric and tartaric acids in particular are preferred for a number of reasons including their high solubility, easy handling, ecological biodegradability, and relatively low cost.

Generally I to 30 grams of color intensifier are dissolved for each 100 grams of cupric carbonate, preferably to grams per 100 grams of cupric carbonate. Amounts of color intensifier greater than per cent by weight based on the cupric carbonate are not helpful and in certain cases may even promote attack of the brass surface by the color intensifier. Amounts of color intensifier less than 1 per cent by weight based on the cupric carbonate produce unsatisfactory deposits in terms of color and uniformity.

The particular shade of brown desired in the coating for a particular application may be attained through judicious selection of an appropriate color intensifier and appropriate bath and operating conditions as hereinafter described. For example, all other factors being equal, malic and adipic acid color intensifiers produce dark brown shades; benzoic, p-hydroxybenzoic and 2- terphthaloyl benzoic acid color intensifiers produce purplish-brown shades; andv salicylic acid, aminoacetic sufficient to provide a pH of about 10.5 to 12.5, and

preferably about 1 1.2 to 12.0. It may be added as ammonium hydroxide reagent or generated in the bath by adding gaseous ammonia.

Color Modifier The color modifiers useful in the practice of the present invention are the thiosulfate and hydrosulfite salts and mixtures thereof. The thiosulfate salts, such as Na S O are preferred over the hydrosulfite salts, such as Na S O because of their greater activity and stability. The salts may utilize any non-interfering cations and, in fact, may be introduced into the bath as acids which will react to produce the appropriate ammonium salts in situ. The alkali metal, copper and ammonium salts are preferred because of solubility, ease of handling and relatively low cost.

Generally from 1 to 30 grams of color modifier are dissolved in each liter of solution and preferably from 4 to 10 grams per liter. Amounts greater than 30 grams per liter are not required while amounts less than 1 gram per liter produce blackish colorations.

Use of the Bath The pH of the bath is maintained in the range of about 10.5 to l2.5'and preferably about 11.2 to 12.0. Values of pH below 10.5 may result in a mottled coating, while values of pH above 12.5 may cause attack of the workpiece. Ease of handling and maintenance suggest the use of a pH level of about 12.0.

The bath is maintained at a temperature of from 20 to 50 Centigrade, preferably at ambient temperatures of from 25 to 30 Centigrade.- Mild agitation of the bath is recommended to insure dissolution of the bath components and a good flow of the bath about the workpiece surface to be darkened.

The workpieces are immersed in the bath for periods of from about 0.5 to 4.5 minutes at ambient temperature to develop the desired brown finish; Immersion times of about 1 to 2 minutes are preferred for the development of light browns, and 2 to 4 for dark browns, although shorter time periods may be employed, particularly at elevated temperatures or with good agitation.

The coating developed on the brass surface can be varied from a light, soft brown to a hard, dark brown by variations in the bath composition and the operating conditions of the bathwithin the specified ranges. Preferably once a particular color intensifier is selected, variation in shade is obtained by varying the immersion time. Extremely elevated bath temperatures and/or very high pH conditions increase the likelihood of attach on the brass surface and are not recommended. While the use of mild bath agitation promotes the uniformity and depth of the deposit, air agitation and excessive agitation which leads to the formation of eddy currents within the bath must be avoided.

Bath Preparation and Maintenance The bath should be prepared by mixing the cupric carbonate, color intensifier, and color modifier in the designated quantities with water sufficient to provide about one-third of the expected total liquid volume required. When a smooth slurry has formed, sufficient ammonia is added, either as gaseous ammonia or as ammonium hydroxide, to raise the pH to the desired level. Water is then added, with stirring, to bring the solution to the proper operating level. After a one or two hour wait for the solution to stabilize the pH is rechecked, and additional ammonia added as necessary. It may be desirable in some instances to add a small amount of a conventional, non-interfering surfactant such as sodium dodecyldiphenyl ether sulfonate, but this is generally not required.

Maintenance of the bath is simple and requires only periodic checking absent excessive contamination. To keep within the desired immersion time over a long period of usage, a first correction. may be effected by gradually increasing the temperature up to the high limit of the preferred range (30 Centigrade). If an excessive immersion time is still required, a replenishment of the bath with several grams of color modifier per liter of bath should be made. After about 246 square decimeters of brass have been treated per liter of bath solution, replenishment may not suffice to produce the desired finish within a reasonable immersion time and in such a case the bath should be discarded and the processing tank recharged. Periodically, any precipitate found at the bottom of the bath should be removed.

Illustrative of the present invention are the following specific examples wherein all parts are by weight unless otherwise specified.

EXAMPLE ONE A bath is prepared by mixing with 350 parts water 64 parts of cupric carbonate, 16 parts of citric acid as a color intensifier, and parts of sodium hydrosulfite as a color modifier. The bath pH is adjusted to 12.5 i0.1 with ammonium hydroxide and the volume of the bath brought up to 1000 parts. The bath is mildly agitated to insure thorough dissolution and mixing of the ingredients, and after a two hour delay to permit the bath to stabilize the pH is readjusted. Brass workpieces are immersed in the bath for 3-4 minutes, the temperature of the bath being determined as 25 30 Centigrade. After being rinsed with hot water and dried, the workpieces have a uniform coatingof a light brown. Prior to drying, the brown coating is rubbed with the index finger and not dislodged, thus indicating a high degree of adherence.

As a control, another brass workpiece is immersed in a bath of similar composition which omits only the color modifier. The coating produced thereby was a dark black.

, EXAMPLE TWO A bath is prepared by mixing with350 parts water 60 parts of cupric carbonate, parts of tartaric acid as a color intensifier, and 4 parts of sodium thiosulfate as a color modifier. The bath pH is adjusted to 12.0 with ammonium hydroxide and the volume of the bath brought up to 1000 parts. The bath is mildly agitated to insure thorough dissolution and mixing of the ingredients, and after a one hour delay to permit the bath to stabilize the pH is readjusted. Brass workpieces are immersed in the bath for 1 2 minutes at a temperature of 25 Centigrade with mild agitation. After being rinsed with hot water and dried, the workpieces have a uniform coating of a rich brown. The adherency of the coating is also found to be excellent.

Thus, it can be seen from the detailed specification and the foregoing specific examples that the present invention provides anovel method and bath for imparting a brown surface coating to brass workpieces which is characterized by the use of a copper carbonate-based bath which may be operated at ambient temperatures and which leaves no smut on the treated surface. The

proper pH and immersion time ranges are relatively stable and the coating produced by the bath is adherent and uniform in appearance and may be selected to be a desired shade of brown by judicious selection of the color intensifier and operating parameters.

Having thus described the invention, I claim:

1. An aqueous bathfor imparting a brown surface coating to brass workpieces comprising an aqueous solution of about 35 to 135 grams per liter of cupric carbonate; about 1 to 30 grams per 100 grams of cupric carbonate of an organic color intensifier selected from the group consisting of aliphatic and aromatic carboxylic acids, aliphatic and aromatic aldehydes, aliphatic and aromatic ketones, aliphatic and aromatic amines, aliphatic alchols. oxidation-resistant hydroxybenzene compounds, aromatic phosphonic and sulfonic acids, and mixtures thereof; about 1 to 30 grams per liter of a color modifier selected from the group consisting of thiosulfate and hydrosulfite salts of non-interfering cations, and mixtures thereof; and ammonium hydroxide in an amount sufficient to provide a pH of about 10.5 to 12.5.

2. The bath of claim 1 wherein said solution contains about 45 to grams of said cupric carbonate per liter.

3. The bath of claim 1 wherein said solution contains about 15 to 25 grams of said organic intensifier per grams of cupric carbonate.

4. The bath'of claim 1 wherein said solution has a pH of about 11.2 to 12.0.

5. The bath of claim 1 wherein said organic color intensifier is a carboxylic acid.

6. The bath of claim 5 wherein said organic color intensifier is an aliphatic polycarboxylic acid selected from the group consisting of tartaric acid, citric acid, and mixtures thereof.

7. The bath of claim 1 wherein said cation of said color modifier is selected from the group consisting of alkali metals, ammonium and copper.

'8. The bath of claim 1 wherein said color modifier is present in an amount of about 4 to 10 grams per liter.

9. The bath of claim 1 wherein said color modifier is a thiosulfatesalt of a cation selected from the group consistng of alkali metals, ammonium and copper.

10. The bath of claim 1 wherein said solution contains about 45 to 75 gramsper liter of cupric carbonate, about 15 to 25 grams per 100 grams of cupric carbonate of tartaric acid as said color intensifier and about 4 to 10 grams per liter of a thiosulfate salt as said color modifier, wherein said solution has a pH of about 1 1.2 to 12.0.

11. The method of imparting brown surface finish to brass comprising the steps of:

dissolving in water reagents to form an aqueous solution of about 35 to grams per liter of cupric carbonate, about 1 30 grams per 100 grams of cupric carbonate of an organic color intensifier selected from the group consisting of aliphatic and aromatic carboxylic acids, aliphatic and aromatic aldehydes, aliphatic and aromatic ketones, aliphatic and aromatic amines, aliphatic alcohols, oxidation-resistant hydroxybenzene compounds, aromatic phosphonic and sulfonic acids, and mixtures thereof; about 1 to 30 grams per liter of a color modifier selected from the group consisting of thiosulfate and hydrosulfite salts of non-interfering cat- 13. The method of claim 11 wherein said solution contains about 45 to 75 grams of said cupric carbonate per liter.

14. The method of claim 11 wherein said solution contains about 15 to 25 grams per 100 grams of cupric carbonate of said organic intensifier.

15. The method of claim 11 wherein said solution has a pH of 11.2 to 12.0.

about 11.2 to 12.0.

16. The method of claim 11 wherein said organic color intensifier is a carboxylic acid.

17. The method of claim 16 wherein said organic color intensifier is an aliphatic polycarboxylic acid selected from the group consisting of tartaric acid, citric acid and mixtures thereof. I

18. The method of claim 11 wherein said color modifier is present in an amount of about 4 to 10 grams per liter.

19. The method of claim 11 wherein said cation of said color modifier is selected from the group consisting of alkali metals, ammonium and copper.

20. The method of claim 11 wherein said color modifier is' a thiosulfate salt of a cation selected from the group consisting of alkali metals, ammonium and copper.

21. The method of claim 11 wherein said solution contains about 45 to grams per liter of cupric carbonate, about 15 to 25 grams per grams of cupric carbonate of tartaric acid as said color intensifier and about 4 to 10 grams per liter of a thiosulfate salt as said color modifier, and wherein said solution has a pH of 

2. The bath of claim 1 wherein said solution contains about 45 to 75 grams of said cupric carbonate per liter.
 3. The bath of claim 1 wherein said solution contains about 15 to 25 grams of said organic intensifier per 100 grams Of cupric carbonate.
 4. The bath of claim 1 wherein said solution has a pH of about 11.2 to 12.0.
 5. The bath of claim 1 wherein said organic color intensifier is a carboxylic acid.
 6. The bath of claim 5 wherein said organic color intensifier is an aliphatic polycarboxylic acid selected from the group consisting of tartaric acid, citric acid, and mixtures thereof.
 7. The bath of claim 1 wherein said cation of said color modifier is selected from the group consisting of alkali metals, ammonium and copper.
 8. The bath of claim 1 wherein said color modifier is present in an amount of about 4 to 10 grams per liter.
 9. The bath of claim 1 wherein said color modifier is a thiosulfate salt of a cation selected from the group consistng of alkali metals, ammonium and copper.
 10. The bath of claim 1 wherein said solution contains about 45 to 75 grams per liter of cupric carbonate, about 15 to 25 grams per 100 grams of cupric carbonate of tartaric acid as said color intensifier and about 4 to 10 grams per liter of a thiosulfate salt as said color modifier, wherein said solution has a pH of about 11.2 to 12.0.
 11. The method of imparting brown surface finish to brass comprising the steps of: dissolving in water reagents to form an aqueous solution of about 35 to 135 grams per liter of cupric carbonate, about 1 30 grams per 100 grams of cupric carbonate of an organic color intensifier selected from the group consisting of aliphatic and aromatic carboxylic acids, aliphatic and aromatic aldehydes, aliphatic and aromatic ketones, aliphatic and aromatic amines, aliphatic alcohols, oxidation-resistant hydroxybenzene compounds, aromatic phosphonic and sulfonic acids, and mixtures thereof; about 1 to 30 grams per liter of a color modifier selected from the group consisting of thiosulfate and hydrosulfite salts of non-interfering cations and mixtures thereof; and ammonium hydroxide in an amount sufficient to provide a pH of about 10.5 to 12.5; maintaining said solution at a temperature of about 20* to 50* Centigrade; and immersing in said solution a workpiece having a brass surface for a period of time sufficient to develop a brown finish thereon.
 12. The method of claim 11 wherein said solution is maintained at a temperature of 25* to 30* Centigrade.
 13. The method of claim 11 wherein said solution contains about 45 to 75 grams of said cupric carbonate per liter.
 14. The method of claim 11 wherein said solution contains about 15 to 25 grams per 100 grams of cupric carbonate of said organic intensifier.
 15. The method of claim 11 wherein said solution has a pH of 11.2 to 12.0.
 16. The method of claim 11 wherein said organic color intensifier is a carboxylic acid.
 17. The method of claim 16 wherein said organic color intensifier is an aliphatic polycarboxylic acid selected from the group consisting of tartaric acid, citric acid and mixtures thereof.
 18. The method of claim 11 wherein said color modifier is present in an amount of about 4 to 10 grams per liter.
 19. The method of claim 11 wherein said cation of said color modifier is selected from the group consisting of alkali metals, ammonium and copper.
 20. The method of claim 11 wherein said color modifier is a thiosulfate salt of a cation selected from the group consisting of alkali metals, ammonium and copper.
 21. The method of claim 11 wherein said solution contains about 45 to 75 grams per liter of cupric carbonate, about 15 to 25 grams per 100 grams of cupric carbonate of tartaric acid as said color intensifier and about 4 to 10 grams per liter of a thiosulfate salt as said color modifier, and wherein said solution has a pH of about 11.2 to 12.0. 